1. Field of the Invention
The present invention relates to a display device including an element having a thin film of a chemical compound capable of electroluminescence (EL) (which film hereinafter referred to as xe2x80x9cEL filmxe2x80x9d) (which element hereinafter referred to as xe2x80x9cEL elementxe2x80x9d), and to a method of fabricating the display device.
EL comprises phosphorescence caused at the time of transition from a triplet excited state to a ground state and fluorescence caused at the time of transition from a singlet excited state to a ground state.
An inorganic material or an organic material may be used to form an EL film. xe2x80x9cOrganic EL filmxe2x80x9d refers to a film of an organic material formed as an EL film. xe2x80x9cOrganic EL elementxe2x80x9d refers to an EL element having an organic EL film interposed between electrodes.
In this specification, xe2x80x9cthin-film transistor (TFT) devicexe2x80x9d refers to a semiconductor device having at least three electrodes. These electrodes comprise a gate electrode, a source electrode, and a drain electrode. Each of the source electrode and the gate electrode may also serve a wiring function in some case.
2. Description of the Related Art
Display devices using an organic EL film can be reduced in weight and thickness in comparison with conventional CRTs, and application of such display devices to various uses is being advanced. Portable telephones and personal portable information terminals (personal digital assistants: PDAs) capable of being connected to the Internet have been developed and the amount of information to be visualized on a display device has become markedly large. Under these circumstances, there is an increased demand for multicolor and/or high-resolution display devices.
As a means to achieve improved resolution of a display device, a method is adopted in which voltage is applied to an EL film by means of active elements such as thin-film transistors (TFTs).
Display devices having pixel portion formed of EL elements are of a self-light-emitting type and therefore require no such light source as a backlight used for liquid crystal display devices. EL elements are therefore considered to be a promising means for realizing lightweight thin display devices.
EL elements are ordinarily constructed in such a manner that an EL film is formed over anodes formed in correspondence with pixels, and a cathode is formed as a common electrode on the EL film. In EL elements constructed in such a manner, however, a discontinuity can be easily caused in the EL film at a side surface of the anode, because the thin EL film having a thickness of 30 to 150 nm is formed over the anode having an increased thickness of about 200 nm, which is selected to set a low resistance. If a discontinuity is caused in the EL film, the anode and the cathode are short-circuited, light emission from the corresponding EL film portion is thereby made impossible, and a black-dot defect results.
A structure such as shown in section in FIG. 18 has been proposed by considering this problem. FIG. 18 is a cross-sectional view of a conventional EL element. An end of an anode 1000 is covered with an insulating film 1001 for the purpose of preventing discontinuity in an EL film 1002 and, hence, short-circuiting between the anode and a cathode 1003. The insulating film provided at the end of the anode is generally called a bump.
The structure shown in the cross-sectional view of FIG. 18, however, entails several problems in an actual fabrication process. If the side surface of insulating film 1001 is linear as shown in FIG. 18, discontinuity is liable to occur in the EL film at a joint 1004 between the upper surface of the anode and the side surface of the insulating film. That is, failure of deposition of the material of EL film 1002 occurs in a place where the gradient of the film forming surface of the EL film changes abruptly, thereby forming a gap in the EL film. Through this gap, the anode and the cathode are short-circuited. Even if discontinuity does not occur in the EL film, the thickness of the EL film may be reduced at the joint 1004 between the upper surface of the anode and the side surface of the insulating film. In such a case, an electric field is converged at the thinner portion of the EL film, so that light emission is caused only at the thinner portion.
In a case where the cathode on the insulating film is electrically connected to a wiring below the insulating film via a contact hole formed through the insulating film, discontinuity in the cathode at the side surface of the insulating film may cause failure to apply the necessary potential for display to the cathode.
Also, discontinuity is liable to occur in the EL film and in the cathode in the vicinity 1005 of the line at which the side and upper surfaces of the insulating film 1001 meet each other. Ordinarily, the insulating film (bump) is in the from of a strip such as to cover the space between adjacent pixel portion. If, on the bump formed so as to surround one pixel portion, a discontinuity is formed in the cathode and extends continuously to form a closed curve, then the portion of the cathode within the closed curve serves no electrode function and no voltage is applied to the EL film. That is, a point defect results.
In manufacture of display devices using EL elements in which the number of pixels is increased to improve the resolution, the occurrence of point defects due to short-circuiting between the anodes and cathode or due to discontinuity in the cathode is a factor which causes a reduction in yield or in image quality, and there is an urgent need to cope with this problem. Also, convergence of the electric field due to the state of the EL film locally reduced in thickness makes the luminance of the defective pixel portion different from that of nondefective pixel portion, resulting in a reduction in viewability. It is also necessary to cope with this problem.
An object of the present invention is to provide a display device in which occurrence of discontinuity in an EL film or an electrode is prevented by smoothly forming the film forming surface, and a method of fabricating the display device.
The inventors of the present invention conceived that a method of smoothly changing, through optimization of the shape of the bump, the gradient of the film forming surface of the bump on which the EL film and the cathode are formed could be effective in improving the facility with which the EL film and the cathode are formed so as to be uniform in thickness and in reducing the occurrence of discontinuity in each of the EL film and the cathode as well as in reducing the amount by which the EL film is locally reduced in thickness. The inventors then optimized the shape of the bump to enable the EL film and the cathode to be formed so as to be uniform in thickness and to achieve improved display performance.
Terms used to express the shape of the bump in the description of the present invention will be described with reference to FIGS. 20A and 20B, which are cross-sectional views of bumps showing examples of bump profiles.
For example, of the bump having a flat upper surface 107 as shown in the cross-sectional view of FIG. 20A, the opposite ends of a bottom portion of an insulating film 101 will be referred to as bottom end portions 104; the opposite ends of a top portion of the insulating film will be referred to as top end portions 106; and portions of the surfaces which contact the upper surface 107 of the insulating film and the upper surfaces of anodes 100 existing below the insulating film, which portions have an intermediate height between the heights of the upper surface 107 and the anode surfaces, will be referred to as central portions 105. The surface of the insulating film is divided into the flat upper surface 107 and side surfaces 108.
For example, of the bump having a curved upper surface as shown in the cross-sectional view of FIG. 20B, the opposite ends of a bottom portion of an insulating film 201 will be referred to as bottom end portions 204; a portion of the insulating film about a highest point will be referred to as a top portion 206; and portions of the surfaces which contact the top portion 206 of the insulating film and the upper surfaces of anodes 200 existing below the insulating film, which portions have an intermediate height between those of the top portion 206 and the anode surfaces, will be referred to as central portions 205.
FIG. 1A shows an example of an arrangement in accordance with the present invention. FIG. 1A is a cross-sectional view of an EL element. In the EL element, an insulating film (bump) 101 is selectively formed on an electrode, e.g., anode 100 on one side. EL film 102 is formed on the insulating film and the anode, and a cathode 103 is formed on the EL film. Features of the present invention reside in the shape of the insulating film. The shape of the insulating film will be described with reference to FIG. 2, which is a cross-sectional view showing the sectional profile of the bump.
Note that in the present invention, the thickness (T) of the insulating film 101 is the film thickness of the insulating film when used in a device, and corresponds to the length of a line defined between the upper and lower surfaces of the insulating film and perpendicular to these surfaces.
From the viewpoint of prevention of discontinuity in each of the EL film 102 and the cathode 103, it is preferred that the thickness of the insulating film be not excessively thick, for example, it should be 3.0 xcexcm or less. Also, it is preferable to set the thickness of the insulating film to a value not smaller than 1.0 xcexcm in order to reduce the parasitic capacitance between the cathode 103 formed on the insulating film 101 and a TFT device formed below the insulating film 101. That is, the thickness of the insulating film is preferably within the range from 1.0 to 3.0 xcexcm.
(1) The present invention is characterized in that, in an EL element having an electrode, e.g., anode 100 on one side and insulating film 101 selectively formed on the anode 100, the bottom end portion 104 of the insulating film contacting the upper surface of the anode contacts an ellipse or circle having its center located outside the side surface 108 of the insulating film, and the top end portion 106 connects continuously to the upper surface 107 of the insulating film and contacts an ellipse or circle having its center located inside the side surface 108 of the insulating film (see FIG. 2B). If the lower end portion and the top end portion of the insulating film are formed smoothly as described above, the gradient of the film forming surface changes continuously, so that the occurrence of discontinuity in each of the EL film 102 and the cathode 103 can be prevented. Also, the amount by which the EL film is locally reduced in thickness in the region between the cathode and the anode can be limited, thereby preventing localized convergence of an electric field in the EL film.
The center of the ellipse is the point of intersection of the minor axis and the major axis of the ellipse. The center of the circle is the point of intersection of at least three radial lines perpendicular to lines tangent to the circle at different positions.
(2) In the arrangement described in (1) above, if the central portion 105 of the insulating film has a side surface contacting the side surface of the insulating film forming an angle xcex8 within the range from 35xc2x0 to 70xc2x0 from the upper surface of the anode, the occurrence of discontinuity in each of the EL film and the cathode on the side surface 108 of the insulating film can be prevented. In this specification, xe2x80x9ccentral portionxe2x80x9d refers to a portion of the surface of the insulating film 101 contacting the upper surface of the insulating film and the upper surface of the anode, which portion has an intermediate height between the height of the upper surface of the insulating film and the height of the upper surface of the anode. In this specification, a surface contacting the side surface of the insulating film will be referred to as xe2x80x9csloped surfacexe2x80x9d, and the angle formed between the sloped surface and the upper surface of the anode will be referred to as xe2x80x9csloped surface taper anglexe2x80x9d.
Preferably, the sloped surface taper angle of the central portion of the insulating film is within the range from 35xc2x0 to 70xc2x0. If the sloped surface taper angle exceeds 70xc2x0, the thickness of the cathode along the side surface of the insulating film becomes excessively small and the possibility of occurrence of discontinuity in the cathode is increased. If the sloped surface taper angle is smaller than 35xc2x0, the insulating film (bump) tends to become smaller in thickness as the sloped surface taper angle is reduced. If the film thickness of the insulating film is reduced, the parasitic capacitance between the TFT device below the insulating film and the cathode on the insulating film becomes disadvantageously large.
(3) According to the present invention, an EL element has an electrode, e.g., anode 100 on one side and insulating film 101 selectively formed on the anode 100. The bottom end portion 104 of the insulating film contacts the upper surface of the anode 100 and contacts a curved side surface determined by a center of curvature (O1) and a first radius of curvature (R1) above a tangential line along which the anode and the bottom end portion contact each other. The top end portion 106 of the insulating film connects continuously to the upper surface of the insulating film and has a curved side surface determined by a center of curvature (O2) and a second radius of curvature (R2) below the boundary between the top end portion 106 and the flat upper surface 107 (see FIGS. 2A and 2B).
The bottom end portion of the insulating film has a curved profile such that the gradient of the EL film forming surface changes continuously. The coverage of the EL film formed on the bottom end portion of the insulating film is thereby improved to enable prevention of discontinuity in the EL film on the bottom end portion. The possibility of short-circuiting of the anode and the cathode due to discontinuity in the EL film is reduced thereby. Also, the EL film can be prevented from being locally reduced in thickness. As a result, localized convergence of an electric field in the EL film can be prevented.
In the top end portion 106 of the insulating film, the gradient of a surface contacting the side surface of the insulating film changes continuously with respect to the upper surface of the anode 100. Therefore, the occurrence of discontinuity in each of the EL film and the cathode in the vicinity of the boundary between the upper surface 107 and the side surface 108 of the insulating film can be prevented. In particular, since the occurrence of discontinuity in the cathode can be prevented, it is possible to prevent the occurrence of a point defect which may be caused when a discontinuity in the cathode extends continuously to form a closed curve in a case where the insulating film is formed so as to fully cover the ends of the anode. Also, in a case where the insulating film is formed into a strip such as to cover a part of an end portion of the anode, it is possible to prevent the wiring resistance of the cathode from being increased by a discontinuity in the cathode. Further, it is possible to reduce the possibility of occurrence of discontinuity in the cathode at a side surface of a contact hole formed through the insulating film in a case where the cathode is brought into contact with a wiring below the insulating film via the contact hole.
(4) The present invention is also characterized in that, in the arrangement described in
(3) above, the first radius of curvature is within the range from 0.2 to 3.0 xcexcm. If the first radius of curvature (R1) is smaller than 0.2 xcexcm, the side surface of the insulating film 101 contacting the anode 100 rises so steeply as seen in the profile that a problem of difficulty in forming the EL film and the cathode uniformly in thickness on the side surface of the insulating film 101 arises. For example, the gradient of the EL film forming surface changes so abruptly that the thickness of the EL film on the portion of film forming surface corresponding to the abrupt change in the gradient is reduced to cause localized convergence of an electric field in the corresponding portion of the EL film. If the first radius of curvature (R1) exceeds 3.0 xcexcm, the area of a thinner portion of the insulating film is so large that there is a tendency to increase the difficulty in suitably covering the TFT device with the insulating film.
When the first radius of curvature is within the range from 0.2 to 3.0 xcexcm, profile control can easily be performed in either of etching using an aqueous solution of an acid or a base and etching using a reactive gas.
(5) Preferably, in the arrangement described in (3) and (4) above, the sloped surface taper angle xcex8 of the central portion 105 of the insulating film is within the range from 35xc2x0 to 70xc2x0.
(6) Preferably, in the arrangement described in (3), (4) and (5) above, the second radius of curvature (R2) is within the range from 0.2 to 3.0 xcexcm. If the second radius of curvature (R2) is excessively small, the side surface of the insulating film contacting the upper surface of the insulating film 101 is curved so sharply that the effect of preventing discontinuity in the EL film is low even though the top end portion has a curved surface. Therefore it is necessary that the second radius of curvature be at least 0.2 xcexcm.
It is suitable to set the second radius of curvature within the range from 0.2 to 3.0 xcexcm as a radius of curvature controllable in either of etching using an aqueous solution of an acid or a base and etching using a reactive gas in an actual fabrication process.
The radius of curvature or the gradient of each of the bottom end portion, the central portion and the top end portion of the insulating film is set within the above-described range to smoothly form the side surface profile of the insulating film as a whole, thereby improving the reliability with which discontinuity in each of the EL film and the cathode is prevented. It is also possible to limit the amount by which the EL film is locally reduced in thickness on the side surface of the bottom end portion of the insulating film and to thereby prevent convergence of an electric field in the thinner portion of the EL film.
FIG. 1B shows the structure having improved reliability with which discontinuity in the cathode is prevented in comparison with the structure shown in FIG. 1A. Referring to FIG. 1B, insulating film 201 is selectively formed on electrodes, e.g., anodes 200, EL film 202 is formed on the insulating film 201, and cathode 203 is formed on the EL film. The structure shown in FIG. 1B is characterized in that the side surface of the insulating film including that of the top portion of the insulating film is curved.
The sectional profile of the insulating film shown in FIG. 1B will be described in detail with reference to FIGS. 3A and 3B.
The thickness (T) of the insulating film shown in FIG. 3A is the length of a line defined between the top end portion of the insulating film and the bottom surface of the insulating film and perpendicular to the bottom surface. The top end portion has a point on the surface of the insulating film at which the distance between the insulating film surface and the plane on which the insulating film is formed is maximized. Preferably, the thickness of the insulating film is 3.0 xcexcm or less.
(7) The present invention is also characterized in that, in an EL element having an electrode, e.g., anode 200 on one side and insulating film 201 selectively formed on the anode, the bottom end portion 204 of the insulating film contacting the upper surface of the anode has a side surface contacting an ellipse or circle having its center located outside the side surface of the insulating film, and the top end portion 206 contacts an ellipse or circle having its center located inside the side surface of the insulating film (see FIG. 3B).
(8) Specifically, in the arrangement described in (7) above, the sloped surface taper angle of the central portion 205 of the insulating film is within the range from 35xc2x0 to 70xc2x0.
(9) According to the present invention, an EL element has an electrode, e.g., anode on one side and insulating film 201 selectively formed on the anode 200. The bottom end portion 204 of the insulating film contacts the upper surface of the anode 200 and has a curved side surface determined by a center of curvature (O1) and a first radius of curvature (R1) above a tangential line of the anode and the bottom end portion. The top end portion 206 of the insulating film has a curved side surface determined by a center of curvature (O2) and a second radius of curvature (R2) below the top end portion. It is preferred that while the side surface of each of the bottom end portion and the top end portion of the insulating film is curved, the sloped surface taper angle of the central portion 205 of the insulating film is within the range from 35xc2x0 to 70xc2x0 (see FIGS. 3A and 3B).
(10) In the arrangement described in (9) above, the first radius of curvature (R1) of the bottom end portion 204 is preferably within the range from 0.2 to 3.0 xcexcm. If the first radius of curvature (R1) is excessively small, the side surface of the insulating film 201 contacting the anode 200 rises so steeply as seen in the profile that the effect of preventing discontinuity and localized thinning of the EL film is reduced, even though the bottom end portion of the insulating film 201 is curved as seen in the sectional profile. Therefore it is necessary that the first radius of curvature be at least 0.2 xcexcm. However, if the first radius of curvature is excessively large, the area of a thinner portion of the insulating film is so large that it is difficult to suitably cover the TFT device with the insulating film. Therefore setting the first radius of curvature to an excessively large value should be avoided in an EL display device. The first radius of curvature of the insulating film is preferably not larger than 3.0 xcexcm. If the first radius of curvature (R1) is within the range from 0.2 to 3.0 xcexcm, profile control can be suitably performed in an actual fabrication process.
(11) According to the present invention, in the arrangement described in (8), (9) and (10) above, the top portion 206 of the insulating film has a curved shape determined by the center of curvature (O2) and the second radius of curvature (R2) below the top portion. If the surface on which the EL film and the cathode are to be formed are formed smoothly as described above, discontinuity in the cathode, which may occur when the film thickness of the cathode on the surface of the insulating film is reduced, can be prevented. The second radius of curvature (R2) of the top portion 206 may be determined by considering the distance between the adjacent pair of anodes. The arrangement shown in FIGS. 1B, 3A, and 3B, in which the surface of the top end portion 206 of the insulating film is curved, is effective in preventing discontinuity in the cathode, which may be caused by an abrupt angular change.
Preferably, the insulating film has such a profile that, as shown in FIGS. 2A and 2B and FIGS. 3A and 3B, the sloped surface taper angle xcex8 of the side surface between the bottom end portion and the top end portion (or top portion) changes continuously through the range from 0xc2x0, the angle at the end of the insulating film contacting the electrode, to 70xc2x0. If the insulating film has such a profile, discontinuity in each of the EL film and the cathode and convergence of an electric field caused by localized thinning of the EL film can be reliably prevented.
If an organic EL material is used to form the above-described EL film, direct-current drive and low-voltage drive of the EL film can be performed. A display device of low power consumption can be manufactured by using such organic EL material.
While the arrangements for active matrix display devices have been mainly described, the present invention can be applied to either passive matrix display devices or active matrix display devices, because it is possible to effectively prevent discontinuity in the cathode and the EL film and localized thinning of the film thickness of the EL film by selecting the profile of the insulating film.
The description has been made with respect to the case where the electrode below the insulating film is an anode. However, a cathode may alternatively be formed below the insulating film.